The present invention relates to a structure and method of manufacturing the same, and more specifically to a method of manufacturing a structure having a specially designated shape, especially an accelerometer, using etching techniques being used in the semiconductor field.
As an example of a structure to which the manufacturing method of the present invention would be applied, a capacitive accelerometer having a cantilever beam structure with a small spring constant as shown in FIGS. 1A-1B is well known (Japanese patent application Laid-Open No. 152369/1989). The accelerometer has a three-layered structure as shown in a sectional view of FIG. 1A. Especially, the structure of a middle layer as shown in a plan view of FIG. 1B comprises a cantilever beam 108, one end of which is fixed to a part of a frame 109, and a seismic mass 107 provided at the tip of the cantilever beam 108. The seismic mass 107 and the cantilever beam 108 have a shape completely symmetric in the thickness direction. As shown in FIG. 1A, a top plate 113 and a bottom plate 114 have film electrodes 111, 112 opposite to the sides of the seismic mass 107. The upper surface and the lower surface of the seismic mass 107 create respective electrostatic capacities via the gap to the film electrodes 111, 112 on the top plate 113 and the bottom plate 114. The accelerometer outputs the inertia force on the seismic mass 107 due to acceleration as an electrical signal based on the variation of the electrostatic capacities of the gaps between the top and bottom plates. As seen from FIGS. 1A-1B, the center of gravity 110 of the seismic mass 107 is on the center axis of the cantilever beam 108 and has no cross-sensitivity.
When the accelerometer as shown in FIGS. 1A-1B is manufactured, chemical anisotropic etching of single crystal silicon is used because the dimensions of the structure can be controlled uniformly with high accuracy. A structure processed by chemical anisotropic etching can attain a shape with sharp corners being constituted by a special crystallographic plane, such as the (111) plane or the (100) plane. However, if excessive acceleration is applied, stress concentration occurs at a corner of the fixed end of the cantilever beam 108, resulting in a fracture from the fixed end of the cantilever beam 108.
As shown above, structures processed by chemical anisotropic etching have some problem of fractures due to the stress concentration of corners. Therefore, for example, as described in Japanese patent application laid-open No. 18063/1989, as means for improving the strength of the fixed end of the cantilever beam 108, a method of processing the work by chemical anisotropic etching and then providing curvatures to the sharp corners by chemical isotropic etching has been designed. According to this, the stress occurring at the fixed end of the cantilever beam is dispersed along the curvatures, resulting in the improvement of the fracture strength of the accelerometer.
However, when chemical isotropic etching is used for processing of structures, problems occur in that chemical isotropic etching must be performed not only for parts to be curved but also for parts corresponding to at least the size of the curvatures, and that since the chemical isotropic etching is diffusion-limited etching, the etching quantity is different depending on the position of the wafer. Thus, uniform processing is not possible on the whole surface of wafer. Also, a complication is added thereto in the use of two kinds of etchants together.
Further, when a structure constituted by a cantilever beam 108 with one end to the inside of a frame 109 and other end having a seismic mass 107 as shown in FIGS. 1A and 1B is manufactured, and the thickness of the frame, the cantilever beam and the seismic mass is symmetric with respect to the center axis of the thickness of the frame and the thickness decreases in the order of the frame, the seismic mass and the cantilever beam, problems occur in that it is difficult to carry out resist coating of a stepped surface with large height and pattern exposure. For example, undesirable removal of the resist layer occurs at the stepped surface and the resist thickness cannot be made uniform in upper and lower sides of the stepped surface, and thereby a portion of overexposure is produced and irregularity of etched forms is liable to occur.